A programmable controller (PC) normally has a bus gate circuit connected between a data bus of a CPU and an input/output bus connected to input/output units. When accessing the input/output units, the CPU issues an instruction to open the bus gate circuit. The input/output units comprise a plurality of input units and output units. Signal sources such as limit switches, photo electric switches, and proximity switches are connected to the input units and output devices such a motors, electric-magnetic valves, and pumps are connected to the output units.
In this way, the input/output unit functions as an interface connecting the PC to the equipment to be controlled and constitutes the portion which is subject to the worst electric environment in each component of the PC. Accordingly, it is of high importance to design the PC such that the input/output unit and input/output bus are protected against malfunction caused by external noises.
In the event an external noise appears on the input/output bus during the time when the CPU transmits output data to an output unit or receives input data from an input unit, the input/output data may be adversely affected, thus causing malfunction to the PC.
It is well known that such disturbance of the input/output bus takes place during the operation of the PC provided that the input/output unit is attached to or detached from the connector of the input/output bus. Accordingly, the conventional PC has been designed to prohibit the attachment and detachment of the input/output unit during its operation.
However, it would be very convenient if a failed unit can be replaced under the condition in which other units are normally operated.